1. Field of the Invention
The present invention relates to a receiving device for use in a CDMA communications system.
2. Description of the Related Art
Mobile communications have become popular in recent years. As communication methods, methods such as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), etc. are used. Among these methods, CDMA draws public attention as one of standard communication methods of the next generation.
CDMA is a communication method utilizing a spread spectrum technique. Accordingly, with CDMA, a plurality of users simultaneously share the same frequency by using spread codes unique to the respective users. As the spread codes, for example, a PN (Pseudo Noise) sequence or a Gold code is used. The bandwidth of a spread code is much wider than that of data to be transmitted.
A transmitting device multiplies data to be transmitted by a spread code, and outputs the result of the multiplication. As a result, the frequency bandwidth of a transmission signal is broadened. That is, a spectrum is spread. A receiving device regenerates data by multiplying a reception signal by the same spread code as that used by the transmitting device. If the receiving device multiplies the reception signal by a spread code different from that used by the transmitting device, the receiving device cannot properly regenerate data.
CDMA has the following features: the number of user channels per unit bandwidth can be increased because a plurality of users simultaneously share the same frequency bandwidth; a transmission signal is unsusceptible to fading because the frequency bandwidth of the transmission signal is broadened; and secrecy (confidentiality) of conversation is maintained because data is not properly regenerated if a transmitting device and a receiving device do not use the same spread code.
FIG. 1 is a block diagram showing the configuration of a conventional receiving device. A receiving device 500 generates a demodulation signal by multiplying a received spread signal by a spread code. Normally, however, a plurality of paths having different communications channel lengths exist between transmitting and receiving devices. Therefore, the spread signal input to a receiving device fundamentally includes a plurality of signals transmitted over different paths. The above described state where a plurality of paths exist is sometimes referred to as a multipath environment.
A path detector 501 identifies the signals transmitted over respective paths under a multipath environment. To be more specific, the path detector 501 comprises a matched filter, and obtains a delay profile of a spread signal by continuously detecting the correlation between a received spread signal and a spread code. The delay profile is data representing an amount of a delay (or a phase delay) of each of signals transmitted over different paths. The path detector 501 generates a timing signal based on this delay profile. A despread demodulator 502 generates a demodulation signal by multiplying a received spread signal by a spread code according to the timing signal.
In mobile communications, the delay profile of a communications channel continuously varies as a mobile terminal moves or the environment of a transmission line changes. In this case, the despread demodulator 502 must adjust the timing at which a spread signal is multiplied by a spread code, depending on a change of the delay profile. Accordingly, the path detector 501 periodically monitors the delay profile of a communications channel, generates a timing signal based on the result of the monitoring, and provides the despread demodulator 502 with the generated timing signal. Then, the despread demodulator 502 continues to demodulate a spread signal while adjusting the multiplication timing according to the timing signal.
In recent years, a receiving device for simultaneously receiving signals transmitted over a plurality of different communications channels in a CDMA communications system has been studied. Up to date, however, a specific proposal for realizing this capability has not been made.
To realize this capability, a plurality of receiving devices each composed of the path detector 501 and the despread demodulator 502, which are shown in FIG. 1, may be arranged. Namely, the arrangement of a plurality of sets of a path detector and a despread demodulator allows this capability to be realized. In this case, each path detector detects a delay profile for each communications channel, while a corresponding despread demodulator demodulates a spread signal according to each detected delay profile.
As is well-known to a person having the ordinary skill in the art, however, the circuitry size of a matched filter arranged within a path detector is very large and consumes much power. Therefore, if a plurality of path detectors are arranged within a receiving device, the circuitry size of the receiving device becomes larger and consumes much more power. That is, this problem is an obstacle to the demand for reducing the receiving device in size.